The invention relates in general to filtering and in particular to a method for operating a filter device and to a filter device.
In press filter devices, a filter chamber is formed between at least two adjacent recess plates by the recess of at least one of the recess plates. For filtering, i.e. retaining a solid content of a suspension as a filter cake and allowing a remaining liquid fraction of the suspension to pass as a filtrate, the suspension is charged from a suspension pipe through a filling shoe between two filter cloths inside the filter chamber. After filtering operation of the filter device a clearance between the adjacent recess plates is established by displacing at least one of the adjacent recess plates, for discharging the filter cake and further cleaning or even replacing the filter cloths, if required.
The filling shoe is between the two filter cloths and, in filtering operation, clamps the filter cloths to the respective filter plates in order to avoid the suspension to leak from the filter chamber. However, this clamping must be released in opening the filter device after filtering operation at least in order to allow the filter cloths to be replaced. Two solutions were suggested for allowing the filter shoe clamping the filter cloths to be released in opening the filter device:
It was suggested to attach the filling shoe to one of the filter cloth. However, due to wear and deformation of both the recess plate and the filter cloth, the shoe being attached to the filter cloth is increasingly difficult to position exactly into its cavity before closing the filter device for the following filtering operation.
It was further suggested to mount the filling shoe outside the cloth area to the recess plate by a mechanism, for example kind of a hinge, or even outside the plate area, e.g. to a plate hanger assembly.
Misalignment of the filling shoe, i.e. horizontally and/or vertically wrong positioning as well as distortion causes leakage of the filter device, increasing forces and following damages to cloth, membrane, filling shoe and recess plate. Leakage between the filling shoe and its respective cavity further causes accumulation of the solid content of the suspension behind the filter cloth as well as blocking of pipe areas, and uneven distribution of drying gas and washing liquids.
Furthermore, in open position of the filter device, the filling shoes in the state of the art allow the suspension and its solid content, or even fragments of the filter cake to drain into the contact area between the filling shoe and the recess plate, in particular near the suspension pipe, causing mechanical as well as sealing problems in filtering operation.